1. Field of the Invention
The present invention relates to the field of lasers, and more particularly, to switched lasers.
2. Background Information
Many laser applications require that the laser beam be intermittent or pulsed. Such applications include laser milling where it is desired to move the workpiece relative to the laser beam while the laser beam is not impinging on the workpiece. Different applications require different duty cycles and on-durations of the laser.
A common technique for providing laser switching with a controllable duty cycle is the acousto/optical modulator (AO modulator). Acousto/optical modulators are relatively fast and are able to switch between on and off in about 20 nanoseconds. This is because the laser beam is on all the time and the acousto/optic modulator merely produces a first order diffraction of the beam in order to deflect a portion of the beam into an "on" beam path to turn the beam "on" in the external environment. High quality acousto/optic modulators are capable of on-to-off ratios of approximately 1,000:1 and thus allow some laser light through even in the off position. Less sophisticated acousto/optic modulators may have an on-to-off ratio of only about 100:1 with the result that a substantial quantity of light bleeds through even in the off state. However, the primary disadvantage of using an acousto/optic modulator for switching a laser is the fact that even the best acousto/optic modulators can only switch about 50% of the available laser light into the on-beam. Consequently, the laser must be able to provide an output power which is at least twice the power level required at the workpiece. Unfortunately, there are many applications where a power level between 1 and 3 watts is desired at a workpiece in the ultraviolet portion of the spectrum and the highest power, reliable, cost effective UV lasers which are available provide only 3.5 to 5 watts of laser power. In those situations, the acousto/optic modulator is not a satisfactory switch due to its losses of 50% or more. In general, losses of 50% are considered undesirable for all types of lasers, whether ion or not and whether argon, krypton, HeNe or CO.sub.2 gas based or rod based.
A second alternative for switching a laser is a Q switch. In Q switching, the Q switch is placed within the laser cavity and comprises a cell which is opaque in the absence of an electrical signal, but which becomes substantially transparent upon application of an appropriate electrical signal to the cell. Q switches are used with ion lasers in which the atomic states of the lasing medium are excited, but no light output is provided because the Q switch attenuates any light traveling along the axis of the cavity to a level where multiple traversals of the cavity are prevented. Q switches are susceptible to crystal damage and cell damage at flux densities which are required for many applications. Thus, the use of Q switches in those applications is not desirable. Q switches for ultraviolet lasers are essentially unavailable due to crystal damage susceptibility. Additionally, in high power Q switched lasers, beam blooming can result from thermal instability in Q switch. When the Q switch is triggered to its clear condition by an electrical signal, the light of the laser passes through the Q switch, strikes the mirror at the end of the cavity and retraverses the Q switch, thereby turning the laser action on very rapidly. The laser light further clarifies the Q switch material with the result that an even higher Q is provided. In this situation, almost all of the laser medium is brought out of the excited state and a short, high energy pulse of laser light is provided. Unfortunately, Q switched lasers are restricted to low repetition rates, since the Q switch must return to an opaque condition following the removal of the electrical trigger signal in order to allow the lasing medium to recharge. Typically, Q switched lasers have a maximum repetition rate of about 1 KHz.
Other techniques, such as mechanical shutters and rotating disks with holes in them have been used to switch lasers. Unfortunately, for high repetition rates (in excess of about 10 KHz), such systems are subject to synchronization and wear-out problems. In addition, for high energy lasers, means must be provided to absorb the laser energy while the shutter is closed or the laser is impinging on an opaque portion of the disk.
In lasers such as ion lasers which have mirrors external to the cavity containing the lasing plasma, it is standard practice to provide mechanical adjusting screws for adjusting the mirror angle relative to the optical cavity axis in order to enable mechanical alignment of the optical system to provide maximum efficiency and energy output. This also enables the user to compensate for any changes in adjustment or alignment during shipping or over a period of time.
There is a need for a fast, reliable technique for switching lasers at frequencies from 1 KHz to 60 KHz or higher with controllable duty cycles.